Step 1

In a Dutch oven over medium-high, heat the oil until shimmering. Add the onion, celery, carrots and ½ teaspoon salt; cook, stirring occasionally, until the vegetables are lightly browned, about 5 minutes. Add the garlic and cook, stirring, until fragrant, about 30 seconds. Add the tomato paste and cook, stirring often, until the paste begins to brown and stick to the bottom of the pot, about 2 minutes.

Step 2

Add 6 cups water and scrape up any browned bits. Stir in the lentils, rosemary and pepper flakes. Bring to a simmer, stirring often, then cover and reduce to medium-low and simmer, stirring occasionally, until the lentils are just shy of tender, about 40 minutes.

Step 3

Uncover, increase the heat to medium and cook, stirring occasionally, until the lentils are fully tender and the soup has thickened slightly, 8 to 10 minutes. Taste and season with salt and black pepper. Serve drizzled with additional oil and sprinkled with Parmesan.